As much as it might seem otherwise, we generally don't do these things because we want to make you unhappy or are utterly incompetent at our jobs. The reason we ask for these (often annoying and occasionally apparently pointless) changes is because we have (wait for it!) the benefit of experience. We might not know for certain which covers will look best on the display table or what the absolute perfect on-sale date for your book might be, but we certainly know what probably won't work in terms of everything from overall appearance to release date to promotional placement.
We know what the co-op deadlines and promotions are; we know what season your book will compete best in; we know (historically speaking) what covers have worked and which ones have tanked; we know who your audience is and how to target them; we know how much your book should cost and what format it should be; we're starting to get a pretty good handle on this newfangled "e-book" nonsense (electronic sales chez moi are quite excellent; I'm not sure how publishers elsewhere in the industry are faring). Are we perfect? Absolutely not. But we've done this for awhile now, and we are (again, generally speaking) pretty damn good at it.
So rest assured, mes auteurs, that should you one day know the glory of representation, advances, royalties, and the